1. Field of the Invention
The invention relates to a grinding method of grinding a roller workpiece to be formed into a roller of a rolling bearing, and a grinding apparatus that grinds the roller workpiece.
2. Description of the Related Art
Rollers of a rolling bearing roll on the raceway surface of an inner ring and the raceway surface of an outer ring. Thus, the outer peripheral face of each roller is subjected to finish grinding. Further, the axial end face of each roller, which comes into sliding contact, for example, with a rib of the inner ring, is also subjected to finish grinding. Conventional methods of grinding the outer peripheral face of a roller include infeed centerless grinding in addition to through-feed centerless grinding described in Japanese Patent Application Publication No. 2009-274192 (JP 2009-274192 A).
The infeed centerless grinding is performed as follows: a roller workpiece to be ground is supported from below by a blade (support member); the outer peripheral face of a regulating wheel is brought into contact with the outer peripheral face of the roller workpiece; the regulating wheel is rotated, which causes the roller workpiece to rotate about its central axis; and a grinding wheel is brought into contact with the outer peripheral face of the roller workpiece that is rotating, to grind the outer peripheral face of the roller workpiece. While the outer peripheral surface the roller workpiece is being ground, a reference member is held in surface contact with the axial end face of the roller workpiece. That is, the outer peripheral face of the roller workpiece is ground using the end face of the roller workpiece as the reference surface.
However, because the roller workpiece is formed through forging and heat treatment performed after the forging, the accuracy of its axial end face is low. Thus, when the roller workpiece is rotated with the reference member held in surface contact with the axial end face of the roller workpiece, the roller workpiece may move (although slightly) back and forth in the axial direction due to the runout of the end face. Grinding the outer peripheral face of the roller workpiece in such a state would result in a low finish accuracy of the outer peripheral face. Then, grinding the axial end face of the roller workpiece using the outer peripheral face of the roller workpiece as the reference surface would result in a low finish accuracy of the end face.
Therefore, grinding of a roller workpiece is performed as follows. First, as illustrated in FIG. 9A, an outer peripheral face 92 of a roller workpiece 90 is ground with a first reference member 95 held in surface contact with an axial end face 91 of the roller workpiece 90. In this case, due to the above-described behavior of the roller workpiece 90, that is, the roller workpiece 90's moving back and forth in the axial direction, the finish accuracy of the outer peripheral face 92 becomes low. In FIG. 9A to FIG. 9E, the surface to be ground is indicated by a triangle. When grinding of the outer peripheral face 92 is completed, as illustrated in FIG. 9B, a second reference member 96 is brought into surface contact with the outer peripheral face 92, and the roller workpiece 90 is rotated and the end face 91 is ground using the outer peripheral face 92 as the reference surface. Because the finish accuracy of the outer peripheral face 92 used as the reference surface is low as described above, the finish accuracy of the ground end face 91 of the roller workpiece 90 also becomes low. Therefore, as illustrated in FIG. 9C, the roller workpiece 90 is rotated and the outer peripheral face 92 is ground again with the first reference member 95 held in surface contact with the end face 91 of the roller workpiece 90. Then, as illustrated in FIG. 9D, the roller workpiece 90 is rotated and the end face 91 is ground again using the outer peripheral face 92 as the reference surface. If the accuracy of the outer peripheral face 92 still fails to reach the design value, as illustrated in FIG. 9E, the outer peripheral face 92 of the roller workpiece 90 is ground using the end face 91 as the reference surface.
In this way, a prescribed dimensional accuracy is secured by repeatedly performing the step of grinding the outer peripheral face 92 using the end face 91 of the roller workpiece 90 as the reference surface, and the step of grinding the end face 91 using the outer peripheral face 92 of the roller workpiece 90 as the reference surface.
In the conventional grinding method described above, increasing the dimensional accuracy of the outer peripheral face of the roller requires repetition of grinding of the outer peripheral face 92 and grinding of the end face 91. However, if these grinding steps are repeatedly performed, the number of man-hours increases and works such as interchanging the reference members 95, 96 and rearranging the roller workpiece 90 to be ground are involved in each step, leading to low productivity.